Nanoparticles fabricated from ferroic materials have attracted much scientific interest, drawing contributions from the nanomaterial, ferroelectric, optical, liquid crystal, metamaterial, and photorefractive communities. The simple addition of low concentrations of ferroic nanoparticles to a variety of media can have startling and unexpected benefits. For example, optical studies of liquid crystal colloids doped with ferroic nanoparticles have become a topical subject in which the additions of ferroelectric and ferromagnetic nanoparticles have variously been reported to moderate the phase transition temperatures, to influence the dielectric anisotropy, to affect the electric field induced liquid crystal reorientation Freedericksz transition, and to increase optical diffraction or beam coupling efficiencies. In particular, liquid crystals appear to benefit significantly through the addition of very small quantities of nanoparticles made from ferroelectric source materials. For example, the use of single ferroelectric domain nanoparticles in liquid crystals has been shown to have a profound effect on the electrical Freedericksz transition threshold, as well as increasing the optical two-beam coupling gain in hybrid photorefractive devices. The premise for adding ferroelectric nanoparticles to liquid crystals is that the permanent spontaneous polarization of these materials may lead to an increase in the overall liquid crystal sensitivity to externally applied electric fields.
The influence of ferroelectric nanoparticles on their environment depends intimately on the net strength of the particle dipole moment arising from the ferroelectric domain spontaneous polarizations. The net dipole moment for any given ferroelectric nanoparticle is maximized when the structure becomes single domain. Unfortunately, common production methods such as chemical precipitation and spark plasma production cannot ensure that the resulting nanoparticles have strong ferroelectric dipole moments or that the material is even ferroelectric for smaller size particles, due to the size dependence of the ferroelectric effect.
In general, ferroelectricity is a property of certain materials to have a spontaneous electric polarization that can be reversed by application of an external electric field. When most materials are polarized by an applied electric field, the induced polarization is almost exactly proportional to the applied electric field, a linear polarization. Ferroelectric materials, however, exhibit a nonlinear polarization and also exhibit a nonzero spontaneous polarization even when the applied electric field is zero. This spontaneous polarization can be reversed by an applied electric field. The polarization is dependent not only on the current electric field but also on its history, yielding a hysteresis loop. Thus, the ferroelectric properties of a material may be evaluated from both the characteristic hysteresis loop and the existence and magnitude of a permanent electric dipole moment.
A number of materials are intrinsically ferroelectric. Common examples of intrinsically ferroelectric materials include barium titanate (BaTiO3), lead titantate (PbTiO3), and lead zirconium titanate ((Pb,Zr)TiO3). Owing to their ferroelectric properties, these materials have already found uses in electronic devices including liquid crystal devices. Nevertheless, the intrinsically ferroelectric materials have very specific compositions and optical properties that inherently may limit the full range of applications in which they may be used. For example, in liquid crystal media requiring some degree of refractive index matching among the components of the liquid crystal media, it may not be possible to acquire both the desired ferroelectric effect and the refractive index match from a narrow list of intrinsically ferroelectric materials.
Thus, there remain ongoing needs for versatile ferroelectric materials that may be used in applications such as liquid-crystal media.